This invention relates to deposition and powder formation methods and more particularly to thin films and fine powders.
Thin films and methods for their formation are of crucial importance to the development of many new technologies. Thin films of less than about one micrometer (.mu.m) thickness down to those approaching monomolecular layers, cannot be made by conventional liquid spraying techniques. Liquid spray coatings are typically more than an order of magnitude thicker than true thin films. Such techniques are also limited to deposition of liquid-soluble substances and subject to problems inherent in removal of the liquid solvent.
There are many existing technologies for thin films deposition, including physical and chemical vapor deposition, plasma pyrolysis and sputtering. Collectively, these techniques are usable to produce thin films of many materials for a wide variety of applications, but it is still impossible to generate suitable thin films of many materials, particularly for thermally labile organic and polymeric materials. Some of these known techniques enable deposition of thin films having physical and chemical qualities, such as molecular homogeneity, which are unattainable by liquid spray techniques. Existing thin film technologies are often also inadequate for many applications due to high power requirements, low deposition rates, limitations upon substrate temperature, or the complexity and expense of deposition equipment. Hence, such techniques cannot be used economically to produce thick films or coatings having the same qualities as thin films.
Accordingly, a need remains for a new surface deposition technique, which has the potential of allowing deposition of thin films not previously possible, with distinct advantages compared to existing thin film technologies.
Similar problems and a similar need exists in the formation of fine powders. Highly homogeneous and very fine powders, such as made by plasma processing, involve a very energy intensive process and are, therefore, expensive to make. Vapor chemical processes are also known for use in making very fine powders (e.g., fumed silica) in down to submicron sizes but are very expensive and also limited to very specific combinations of chemical reactants. Mechanical grinding produces particles of irregular shape and wide variation in size, predominantly in a range of about 10-300 .mu.m and with 1 .mu.m constituting the practical minimum size, although a fraction with smaller particles may be produced (due to the wide distribution). It can also be very costly. Preparation of polymer powders by atomizing from a liquid solution, as disclosed in U.S. Pat. No. 4,012,461 to van Brederode, is limited to liquid-soluble polymers having a decomposition point higher than 100.degree. C. It produces 20-30% agglomerates requiring further reduction to produce a particle size yield of 99% less than 100 .mu.m, a minimum size of about 5 .mu.m, and an average size range of 20-30 .mu.m. Another technique for atomizing a mixture of molten, normally-solid polymer and a liquid solvent, disclosed in U.S. Pat. No. 3,981,957 to van Brederode et al. requires a separate blowing gas, e.g., nitrogen and a two-fluid nozzle. It produces particles of a size on the order of less than 200 .mu.m. When feed temperature is maintained sufficiently high, such particles are substantially spherical. Fibers are produced at lower temperatures.
Neither the foregoing nor any other prior process is known to be able to produce powders in an average size range of 1-3 .mu.m or smaller. Nor are the foregoing processes applicable to non-molten or liquid insoluble materials, e.g., inorganic compounds such as solid silica (SiO.sub.2). Moreover, these patents indicate that the powders produced are essentially spherical, which shape provides a minimal surface area. For some applications, e.g., catalytic processes, it is desirable to have fine powders of much greater surface area than provided by spherical powders.
Accordingly, a need also remains for improved methods of forming powders.